The present invention relates to a device for injecting a stream of fluid used as a diverted fluid in a separation process employing a plurality of beds of adsorbant, a main fluid stream and a plurality of secondary fluid streams, the beds being separated by at least one fluid distributor plate, the plate possibly comprising one or more distributor-mixer-extractor panels, DME panels, which can inject and/or mix and/or extract one or more secondary fluids via a single distribution chamber.
The invention is of particular application to the device and process described in the Applicant""s French patent application FR-A-2 772 634, when secondary fluids are injected and extracted by means of a single distribution network, normally termed a distribution xe2x80x9cspiderxe2x80x9d.
The invention also relates to all processes in which the purity of at least one constituent in a mixture moving through a solid adsorbant or solid catalyst is to be improved.
It can also be applied to processes for separating at least one constituent from a mixture for which any chromatographic adsorption or ion exchange separation, for example, is carried out.
Throughout the description, the term xe2x80x9cmain fluidxe2x80x9d designates a stream of fluid that moves through the beds of adsorbant, and the term xe2x80x9csecondary fluidxe2x80x9d designates fluids that are used within the separation process, for example desorbant, feed, extract or raffinate and which pertain to the exterior.
The invention is of particular application to separating n-paraffins containing 10 to 13 carbon atoms from sweetened kerosene type feeds.
The prior art describes different devices and processes that can separate feeds using a simulated moving bed. The following can be cited: U.S. Pat. Nos. 2,985,589, 3,214,247, 3,268,605, 3,592,612, 4,614,204, 4,378,292, 5,200,075, 5,316,821, European patent application 0,769,316, French patent application FR-A-2 772 634 and International patent application WO-A-95 03867.
As a general rule, a simulated moving bed comprises at least three chromatographic zone, advantageously four or five, each zone being constituted by at least one bed or column section.
Between two zones there is either a point for injecting a feed to be fractionated, or a point for injecting an eluent or desorbant, or a point for drawing off an extract between the point for injecting eluent and the point for injecting feed which is located downstream (in the direction of movement of the eluent), or a point for drawing off a raffinate between each point for injecting mixture and the point for injecting eluent which is located downstream in the direction of movement of the eluent.
The assembly of beds or column sections forms a closed loop including at least one flow regulated pump for recycling the main fluid, for example between the first and last section.
During the separation process, the injection points and drawing off points of at least one section or column are generally displaced in the same direction (upstream or downstream, again in the direction of movement of the main fluid). This forms the basis of the simulated moving bed operation.
During that process, it is important for the fluid distribution in each of the beds of adsorbant to be as uniform as possible and as homogeneous as possible.
Distribution over each of the beds necessitates collecting the streams from the preceding bed (main fluid moving along the main axis of the column), the possibility of injecting an accompanying fluid or a secondary fluid while mixing the two fluids to the best possible extent, or the possibility of removing a portion of the collected fluid, or drawing it off to send it outside the device and to re-distribute a fluid over the subsequent bed.
To this end, it is possible either to convey all of the main fluid or stream in the adsorber in accordance with a scheme described in U.S. Pat. No. 2,985,589, or to remove a large portion or all of that stream outside using the process described in U.S. Pat. No. 5,200,075.
As described in patent application FR-A-2 772 634, a further solution consists, of moving the majority of the main stream internally and a minority of the stream externally, typically 2% to 20% of the stream. One advantage of such a system is that the injection and withdrawal circuits for the secondary fluids permanently have substantially the same composition. Two distributor plates are connected via an external circuit known as a synchronous bypass circuit. That circuit circulates a minority of the removed stream, ensuring an identical composition. On-off valves are connected to the bypass circuit for removing secondary fluids, along with a non-return valve. Optionally, the circuit can be provided with an on-off valve or a control valve to inject and draw off from a single plate.
Continuous rinsing of the distribution spiders of the distributor plates for the simulated moving bed separation units can be carried out in two manners:
1) when each plate is provided with at least two independent distribution networks (D1 and D2), network D1 of plate P is brought into communication with network D2 of the plate P+1, and the network D1 of plate P+1 is brought into communication with network D2 of plate P+2, so that on each distributor plate, all of the distribution networks have a permanent fluid circulation, and each plate experiences a flow of a stream of fluid diverted from one distribution network to the main fluid and a second fluid stream diverted in substantially the same manner from the main fluid to the second distribution network. The driving force for such flows is provided by the pressure drop caused by the flow of main fluid in the granular porous medium located between two successive distributor plates;
2) when each distributor plate is provided with a single distribution network, the bypass circuits can only be established in every other bed, for example from plate P to plate P+1, then from plate P+2 to plate P+3. If a bypass line connected plates P+1 to P+2, this would result in parallel circulation to the adsorber from the top bed to the bottom bed. The disadvantage of establishing only one bypass circuit in every other bed is that the internal flow rates would vary from one bed to the other: on beds including a bypass circuit, there would be a flow rate D, while on beds not including a bypass circuit, there would be a flow rate D+b.
A further patent FR-A-2 794 663 described a device for injecting a diverted fluid originating from a distribution, mixing and/or extraction chamber from an upstream plate in a region Ri of an adsorbant bed downstream of that plate.
The present invention concerns a device and process that are particularly suitable for separation devices in which the plates are provided with a single network for distributing secondary fluids, distributor-mixer-extractor or DME panels comprising a single distribution, extraction and/or mixing chamber.
Since users"" demands for quality have increased, the purity standard to be achieved has been raised from values in the range (98.1-99%) to 99.5% for C10 to C13 n-paraffins. As a result, operators have to modify existing separation units to satisfy those new requirements.
The invention can be applied to existing units and to new units. It is used in processes and devices for separation, by adsorption in counter-current simulated moving bed processes and optionally in units comprising a rotary valve. It is applicable, for example, in a device comprising a central distribution described in U.S. Pat. No. 4,378,292, for example, in which the distributor plates are provided with a single distribution spider, when revamping a unit or during a significant change in the composition of the feed which results in a change in the configuration of the unit. The invention is also applicable to debottlenecking a unit.
More generally, the modification supplied by the scheme of the invention is particularly applicable in the case of debottlenecking operations in a unit to replace the molecular sieve and/or to mechanically strengthen the distributor plates. When this type of modification is required, it is accompanied by a change in the existing rotary valve; a change in configuration necessitates replacing the valve rotor, doubling the capacity of the rotary valve by placing a second valve in parallel. These transformations are very expensive and can advantageously be replaced by removing the rotary valve and replacing it with 96 or 120 on-off valves at about half the total cost.
Certain separations can be carried out in chromatographic columns with a sufficiently small diameter not to require a central distributor plate support. Re-injecting the diverted fluid to rinse the distribution lines into a particular zone of the adsorbant bed in accordance with FR-A-2,794,663 can have the disadvantage of thermal shock when the momentarily interrupted movement of said diverted fluid resumes. Such a thermal shock on the mineral adsorbant can result in the formation of fines, causing a substantial loss in performance.
One aim of the present invention is to define where the diverted fluid is injected into smaller diameter columns to overcome this disadvantage.
The invention concerns a device that can separate at least one compound from a mixture or a body by simulated moving bed adsorption, comprising:
at least one vessel or column comprising one or more beds of adsorbant (Ai), two beds of adsorbant being separated by at least one plate (Pi) for distributing and extracting fluids, the plate comprising one or more panels for distributing, mixing and/or extracting fluids;
at least one conduit for introducing a main fluid and at least one conduit for extracting main fluid;
a plurality of conduits for extracting or injecting secondary fluids;
a bypass circuit placing a distributor plate in communication with at least one bypass line (Li,j);
the pannel comprising a single chamber (Ci) for distribution, mixing and/or extraction.
It is characterized in that:
the device comprises means (14, Voi,j, 20) for bringing at least one chamber (Ci) into communication with at least one bypass line (Li,j);
one end of the bypass line is connected with said chamber Ci of a pannel of plate Pi, and the other end communicates with a region (Ri+1) of a pannel of a distributor plate Pi+1, said region being distinct from the distribution chamber Ci+1.
The communication means comprise, for example, at least one valve Voi,j disposed on at least one bypass line (Li,j), and the end of the bypass line that is not connected to the region of the plate can be connected to an introduction and/or extraction line (Ti).
The communication means comprise, for example, at least one rotary valve, said rotary valve being connected to at least one introduction and/or extraction line (Ti) and at least to a bypass line (Li,j), said valve comprising means at least for bringing an introduction and/or extraction line into communication with at least one bypass line.
The rotary valve, an embodiment of which is described in the Applicant""s patent FR-A-2 794 663, can place a plurality of groups of lines in communication, for example group G1, group G2 and group G3, said valve comprising:
a stator provided with a plurality of circulation means (E, F, R, S, D) for the fluid or fluids from group G1, means for passage of at least two fluids F1, F2 from group G3;
a rotor provided with means for passage of fluids from group G3 and means for placing either the fluids from group G1 in communication with group G3, or from group G3 in communication with group G3;
the number of means for passage of fluid F1 is substantially identical to the number of means for passage of fluid F2, said valve comprising means for bringing at least two fluids from group G3 into communication and in that the cross section for passage S1 of ports for fluid F1 is different from the cross section for passage S2 of ports intended for fluid F2.
The valve""s means for passage of fluid F1 and of fluid F2 have, for example, respective cross sections for passage S1 and S2 and the ratio S1/S2 is about 4, preferably in the range 2 to 10.
The means for bringing the fluids from group G3 into communication can be constituted by notches disposed in a layer of material or liner deposited on the lower surface of the rotor.
One notch, for example is of depth xe2x80x9cpexe2x80x9d and the depth is at least equal to the thickness xe2x80x9cexe2x80x9d of the liner.
Circulation means (E, R, S, D, F) disposed in the valve are, for example, formed from a plurality of grooves disposed on the bearing surface or upper face of the stator and the notches are disposed in the liner.
The number of these circulation means is 5, for example.
The fluid distribution circuit is, for example, disposed around said vessel, and it can comprise a main line that is divided into a plurality of secondary lines so that the fluid or fluids reach the panels forming a plate at substantially the same time.
The panels can be parallel to each other and the fluid distribution active principles comprises a main conduit, the bypass line is connected to the region of the plate, for example.
A plate is delimited, for example, by a lower screen and an upper screen and in a variation, the end of the bypass line connected with the distribution chamber of a plate Pi is connected to a distribution means disposed below said upper screen in a collecting space of a downstream plate Pi+1.
In a further variation, the end of the bypass line connected with the distribution chamber of a plate Pi can be connected to a distribution means disposed below the upper screen in a mixing chamber that is contiguous with the distribution chambers of plate Pi+1.
The invention also concerns a process for injecting a diverted fluid in a process for separating constituents of a feed by a simulated moving bed process comprising at least the following steps:
moving a main fluid through a plurality of beds of adsorbant disposed between distributor plates;
injecting and extracting secondary fluids (feed, desorbant, etc.) in a sequence that is appropriate for separating the constituents in the feed that are to be separated;
injecting a fluid diverted from one plate Pi to the subsequent plate (Pi+1).
It is characterized in that at least a portion of the main fluid is moved outside the separation vessel via a bypass line comprising at least two ends, one end being connected with a distribution chamber Ci and the other end being connected with a region of a subsequent plate Pi+1 distinct from a region (Ri+1) of said plate Pi+1, said region being distinct from Ci+1.
As an example, a fraction of the main fluid is removed from one chamber (Ci) corresponding to a plate Pi and that fraction of main fluid is injected into a mixing chamber contiguous with a distribution chamber Ci+1 of the subsequent plate Pi+1.
The diverted fluid can be injected during the whole duration of the cycle period. The flow rate of the diverted fluid is less than 5%, for example, than that traversing the bed located between Pi and Pi+1.
The device and process are applicable, for example, to separating normal paraffins containing 10 to 13 carbon atoms from sweetened kerosene type feeds.